Apparatus and a method for generating droplets

ABSTRACT

An apparatus is described. The apparatus comprises a liquid dispenser comprising a liquid outlet, the liquid dispenser being configured to generate a droplet stream. The apparatus also comprises a first fluid flow device configured to generate a confinement fluid flow for confining a trajectory of the droplet stream therein, the first fluid flow device comprising an outlet arranged to allow the droplet stream to exit therefrom, wherein the fluid outlet is arranged within the confinement fluid flow device. Furthermore, the apparatus comprises a second fluid flow device configured to generate a reaction fluid flow for reacting with droplets in the droplet stream, wherein the outlet of the first fluid flow device is arranged within the second fluid flow device. A method is also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a national stage application in the United States based onInternational Application No. PCT/EP2016/053643, filed Feb. 22, 2016,which claims priority to European Patent Application No. 15382069.1,filed Feb. 20, 2015, the contents of each of which are incorporatedherein by reference in their entirety.

The invention relates to an apparatus and methods for generatingdroplets. The present disclosure relates to a method of generating acollimated stream of droplets from a flow focusing nozzle, and a methodand an apparatus for preserving droplets when immersed (i.e. introduced)in to a drying or hardening environment and/or for protecting dropletsduring formation.

Techniques are known for producing collimated droplet streams, such asflow focusing. The droplets in the droplet streams may be subsequentlydried to form a powder, for example, which is often referred to as spraydrying. However, the number of droplets that are capable of being driedcompared to the number of droplets that are produced is often low (e.g.,below 80%), due to the droplets being attracted, and/or adhering, to theapparatus. Moreover, as the droplets exit from a droplet forming device,the initial properties/characteristics of the droplets may be lost oraffected such that the droplet loses its form and may becomeagglomerated and/or polydispersed, or may not even be produced.

FIG. 1 illustrates an example of a known system 1 that is used for spraydrying. The system 1 includes a droplet forming device 2, such as a flowfocusing nozzle. The illustrated example has been simplified, but itwill be appreciated that the flow focusing nozzle is fed with one ormore fluids 4 to produce a droplet stream at its outlet (not shown). Theoutlet of the device 2 is within a drying chamber, which is in thisbackground example is an elongate cylindrical conduit 6. The conduit 6is provided with an air stream via fluid inlet 8. The air stream is at atemperature greater than room temperature (i.e., 20° C. to 26° C.) suchthat as the droplet stream exits the device 2, the droplets are airdried. However, the air stream provided within the conduit 6 may preventdroplets being effectively produced or, as is mentioned above, mayaffect the droplets such that they become agglomerated and/orpolydispersed.

Accordingly, it is an object of the present invention to improve theyield of droplets and/or powder produced by such spray drying apparatus,and to provide a stabilization environment for droplets exiting adroplet forming device before being air dried.

The present disclosure can be understood with reference to thedescription of the embodiments set out below, in conjunction with theappended drawings in which:

FIG. 1 illustrates schematically an example of a known air dryingsystem;

FIG. 2 illustrates an apparatus for spray drying according to a firstembodiment of the invention;

FIG. 3 illustrates an expanded view of the apparatus illustrated in FIG.2;

FIG. 4 illustrates schematically a cross-section through an apparatusaccording to the first embodiment of the invention;

FIG. 5 illustrates, in an upper image, polydispersed, agglomerateddroplets, and, in a lower image, monodispersed, dried droplets;

FIG. 6 illustrates a partial cross-section through the apparatusaccording to the first embodiment of the invention;

FIG. 7 illustrates a cross-section through a flow focusing nozzleaccording to the first embodiment of the invention;

FIG. 8 illustrates an apparatus for spray drying according to a secondembodiment of the invention;

FIG. 9 illustrates an apparatus for spray drying according to a thirdembodiment of the invention; and

FIG. 10 illustrates a multi-nozzle fluid dispenser which may be usedwith the second and third embodiments of the invention.

According to a first aspect of the invention there is provided anapparatus comprising: a liquid dispenser comprising a liquid outlet, theliquid dispenser being configured to generate a droplet stream; a firstfluid flow device configured to generate a confinement fluid flow forconfining a trajectory of the droplet stream therein, the first fluidflow device comprising an outlet arranged to allow the droplet stream toexit therefrom, wherein the fluid outlet is arranged within theconfinement fluid flow device; and a second fluid flow device configuredto generate a reaction fluid flow for reacting with droplets in thedroplet stream, wherein the outlet of the first fluid flow device isarranged within the second fluid flow device. Thus, the droplets duringformation are protected from the reaction fluid flow and a greaternumber of the droplets produced using the liquid dispenser may bepersevered. Accordingly, at least 99.5% of the droplets that areproduced will exit the first fluid flow device and can be subsequentlytransformed to form particles.

The liquid dispenser is configured to generate the droplet stream usingflow focusing.

The liquid dispenser may be concentrically arranged within the firstfluid flow device (i.e., the liquid dispenser and the first fluid flowdevice may share the same centre or axis, such that they are coaxial)and the first fluid flow device may be concentrically arranged withinthe second fluid flow device.

Each of the confinement fluid and/or the reaction fluid may be air.

The first and/or second fluid flow devices may comprise an elongateconduit and the first and/or second fluid flow devices may comprise acircular conduit (i.e., the conduit has a circular cross section). Theconduits may also be non-circular (e.g. oval, elliptical or simplepolygon). It will be understood that if a non-circular conduit is usedits radius is taken to be half the largest distance between any pair ofvertices.

The liquid outlet may comprise a plurality of nozzles, each configuredto generate a droplet stream.

The temperature of the second fluid flow may be greater than thetemperature of the droplet stream and/or the confinement fluid flow.

The reaction may comprise hardening the droplets in the droplet stream.

The first fluid flow device may be dimensioned to produce a laminarfluid flow, and may be dimensioned to prevent the droplet stream frombeing agglomerated.

According to a second aspect of the invention there is provided a systemcomprising an apparatus according to any one of the above-describedapparatus and a first fluid flow generating device coupled to the firstfluid flow device and configured to provide the confinement fluid to thefirst fluid flow device. The first fluid flow generating device and thefirst fluid flow device may be configured according to the followingexpression:

$U_{\min} \geq {0.2 \cdot \left( \frac{\mu_{g}^{2}H^{3}}{\rho_{g}^{2}D_{1}^{5}} \right)^{1/2}}$

-   -   where: U_(min) is the velocity of the flow rate of the        confinement fluid; μ_(g) is the viscosity of the confinement        fluid; H is the distance between the liquid outlet of the liquid        dispenser and the outlet of the first fluid flow device; ρ_(g)        is the density of the confinement fluid; and D₁ is the external        diameter of the liquid dispenser.

According to a third aspect of the invention there is provided a methodcomprising: generating a droplet stream from a liquid dispenser;generating a confinement fluid flow within a first fluid flow device toconfine the trajectory of the droplet stream, wherein the droplet streamis generated within the confinement fluid flow and exits from an outletof the first fluid flow device; and generating a reaction fluid flowwithin a second fluid flow device for reacting with droplets in thedroplet stream, wherein the confinement fluid flow is generated withinthe reaction fluid flow.

The reaction fluid flow may harden the droplets to formparticles/capsules and the method may comprise collecting theparticles/capsules at an outlet of the second fluid flow device.

A minimum velocity of the confinement fluid flow is represented by theexpression:

$U_{\min} \geq {0.2 \cdot \left( \frac{\mu_{g}^{2}H^{3}}{\rho_{g}^{2}D_{1}^{5}} \right)^{1/2}}$

-   -   where: U_(min) is the velocity of the flow rate of the        confinement fluid; μ_(g) is the viscosity of the confinement        fluid; H is the distance between the liquid outlet of the liquid        dispenser and the outlet of the first fluid flow device; ρ_(g)        is the density of the confinement fluid; and D₁ is the external        diameter of the liquid dispenser.

According to a fourth aspect of the invention there is provided a methodof generating a collimated stream of droplets from a flow focusingnozzle comprising a discharge orifice, a first fluid dispenser and asecond fluid dispenser, wherein the second fluid dispenser is arrangedto accelerate, with a carrier fluid, a fluid being dispensed from thefirst fluid dispenser out of the discharge nozzle, the method comprisingconfiguring the flow focusing nozzle to obtain a geometric standarddeviation of the disbursement of droplets from the collimated stream ofdroplets of less than 1.6 according to the expression:

$\left( \frac{D^{2}\Delta\; P^{2}\rho_{l}^{2}Q_{l}}{\sigma\;\mu_{l}^{3}} \right)^{1/4} \leq \frac{1 + {0.5\left( \frac{\rho_{l}}{\rho_{d}} \right)}}{1 + {0.018\left( \frac{\mu_{l}}{\mu_{d}} \right)}}$

-   -   where D is the nominal diameter of the discharge orifice, ΔP is        the pressure of the carrier, ρ₁ is the density of the fluid        being dispensed, Q_(l) is the flow rate of the fluid being        dispensed from the first fluid dispenser, σ is the surface        tension between the fluid being dispensed and the carrier fluid,        μ_(l) is the viscosity of the fluid being dispensed, ρ_(d) is        the density of the carrier fluid, and μ_(d) is the viscosity of        the carrier fluid.

The flow focusing nozzle may be configured to obtain a geometricstandard deviation of the disbursement of droplets from the collimatedstream of droplets of less than 1.4, or less than 1.3.

The first fluid dispenser may comprise an inner fluid dispenserconfigured to dispense a core fluid and an outer fluid dispenserconfigured to dispense a shell fluid and wherein ρ_(l), Q_(l), and μ_(l)are of the shell fluid.

According to a fifth aspect of the invention there is provided aparticle manufactured according to any one of the above-describedmethods.

According to a sixth aspect of the invention there is provided a methodfor designing an apparatus comprising: providing a liquid dispensercomprising a liquid outlet, the liquid dispenser being configured togenerate a droplet stream; providing a first fluid flow deviceconfigured to generate a confinement fluid flow for confining atrajectory of the droplet stream therein, the first fluid flow devicecomprising an outlet arranged to allow the droplet stream to exittherefrom, wherein the fluid outlet is arranged within the confinementfluid flow device; providing a second fluid flow device configured togenerate a reaction fluid flow for reacting with droplets in the dropletstream, wherein the outlet of the first fluid flow device is arrangedwithin the second fluid flow device;

and configuring the liquid dispenser and the generated confinement fluidflow to achieve a minimum velocity of the confinement fluid flowaccording the expression:

$U_{\min} \geq {0.2 \cdot \left( \frac{\mu_{g}^{2}H^{3}}{\rho_{g}^{2}D_{1}^{5}} \right)^{1/2}}$

-   -   where: U_(min) is the velocity of the flow rate of the        confinement fluid; μ_(g) is the viscosity of the confinement        fluid; H is the distance between the liquid outlet of the liquid        dispenser and the outlet of the first fluid flow device; ρ_(g)        is the density of the confinement fluid; and D₁ is the external        diameter of the liquid dispenser.

FIG. 2 illustrates an apparatus 10 for spray drying. The apparatus 10can be separated into three main parts; a liquid dispenser or liquiddispensing device 12, a first fluid flow device orconfinement/prevention device 14 and a second fluid flow device orreaction/drying device 16. FIG. 3 illustrates the apparatus 10 in anexpended view.

FIGS. 2 and 3 are now used to describe the apparatus 10. The liquiddispenser 12 is a form of flow focusing device or nozzle. However, itwill be appreciated that a flow focusing nozzle is used here as anexample, but other types of nozzle (e.g., vibrating nozzle, ultrasonicnozzle, or ink-jet nozzle) may be used. The operation of the liquiddispenser is described in more detail with reference to FIG. 7. Theliquid dispenser 12 includes two fluid inlets 18, 20 for introducingfluids to produce a droplet at a proximate end. For example, the firstfluid inlet 18 may be fed with a core-droplet fluid, for example, anoily fragrance, and the second fluid inlet 20 may be fed with ashell-droplet fluid, for example, an aqueous polymeric solution (e.g.,Arabic Gum or Shellac). The resultant droplet is in the form of acore-shell or capsule droplet (i.e., a droplet comprising a core and anouter shell). It will be appreciated that the liquid dispenser 12 mayonly include a single fluid inlet (i.e., one of two fluid inlets 18, 20)for feeding only a single fluid for the production of droplets. Othercore or shell fluids include aqueous or organic suspensions, emulsions,organic polymeric solution, aqueous/organic solutions includingingredients, actives, pure liquids or solutions including mixtures ofcomponents such as plasticizers, additives, thickeners, and monomers. Athird fluid inlet 22 is provided for feeding a carrier fluid such asair. Other gases may be used as a carrier fluid, such as argon ornitrogen. The fluid dispenser includes a circular, elongate conduit orduct 24 having an external diameter of 5 mm which is coupled to each ofthe fluid inlets 18, 20, 22 and is terminated at a distal end with anozzle, orifice or outlet 25. The nozzle 25 in this embodiment includesa single outlet orifice having a diameter of 350 μm (not shown). Theduct 24 includes one or more internal ducts or conduits which aredescribed in more detail with reference to FIG. 7.

The first fluid flow device or confinement/prevention device 14 includesan elongate circular conduit or duct 26 having an internal diameter of60 mm. The duct 26 has at its proximal end a cap, lid or cover (i.e., aclosure device) 28 used to close the duct 26 at its proximal end exceptfor an opening or inlet 32 to allow the liquid dispenser 12 to beinserted therein. The diameter of the duct 26 is greater than thediameter of the duct 24. The duct 24 of the liquid dispenser 12 isarranged such that at least a portion (i.e., a distal portion) extendsthrough the opening 32 and within the duct 26, and a further portion(i.e., a proximal portion) extends outside of the duct 26. As can beseen in FIG. 2, the nozzle 25 is arranged part way along the length ofthe duct 26 (i.e., the liquid dispenser is terminated within the duct26). The liquid dispenser 12 is arranged concentrically within the firstfluid flow device 14. The closure device 28 includes an inlet 30 that iscoupled to an internal channel (not shown) to allow the passage of fluidfrom the inlet 30 through the internal channel of the closure device 28.The closure device 28 also includes an opening (not shown) coupled tothe internal channel to allow the passage of a fluid into the duct 26.The opening within the closure device 28 may be circular to match thecross sectional shape of the duct 26. The circular duct 26 has anopening, exit or outlet 34 at its distal end to allow for the passage ofdroplets and the fluid fed to the fluid inlet 30.

The second fluid flow device or reaction device 16 includes an elongatecircular conduit or duct 36 having an internal diameter of 147 mm. Theduct 36 has at its proximal end a cap, lid or cover (i.e., a closuredevice) 38 that closes the duct 36 at its proximal end except for anopening or inlet 42 to allow the duct 26 of the first fluid flow device14 to be inserted therein. The diameter of the duct 36 is greater thanthe diameter of the duct 26. The duct 26 of the first flow device 14 isarranged such that at least a portion (i.e., a distal portion) extendsthrough the opening 42 and within the duct 36, and a further portion(i.e., a proximal portion) extends outside of the duct 36. As can beseen in FIG. 2, the opening 34 is arranged part way along the length ofthe duct 36 (i.e., the first fluid flow device 14 is terminated withinthe duct 36). The first fluid flow device 14 is arranged concentricallywithin the second fluid flow device 16. The closure device 38 includesan inlet 40 that is coupled to an internal channel (not shown) to allowthe passage of fluid from the inlet 40 through the internal channel ofthe closure device 38.

The closure device 38 also includes an opening (not shown) coupled tothe internal channel to allow the passage of a fluid into the duct 36.The opening within the closure device 38 may be circular to match thecross sectional shape of the duct 36. The circular duct 36 has anopening, exit or outlet 44 at its distal end to allow for the passage ofdroplets and the fluid fed to the fluid inlets 30, 40. Alternatively,fluid may be drawn from the outlet 44 by creating a negative pressure atthe outlet 44 using a suitable suction pump.

In this example, the second (reaction) fluid flow device 16 ismanufactured from stainless steel (i.e., the closure device 38) andglass (i.e., the elongate conduit 36), and the first (confinement) fluidflow device 14 is manufactured from similar materials. The liquiddispenser 12 is manufactured from plastics and stainless steel. Glasshas been used in this example to reduce any large scale fluctuationsthat could result from any internal roughness on the interior walls ofthe elongate conduits 26, 36.

FIG. 4 illustrates schematically a cross-section through the apparatus10 illustrated in FIG. 2. The same reference numerals are used in FIG. 4as used in FIGS. 2 and 3 for like features. FIG. 4 illustrates theinternal channel 66 of the closure device 28 and its associated circularopening 68. FIG. 4 also illustrates the internal channel 71 of theclosure device 38 and its associated circular opening 73.

In operation, the apparatus 10 is coupled to a number of feedingsystems, which may include compressors which are configured topressurize air and feed the pressurized air to various inlets of theapparatus 10. The fluid dispenser 12 is coupled to a compressor 50 via apipe (line or passage) 52 at its inlet 22. The compressor 50 in thisexample feeds air to the dispenser 12 so that the air provides a carrierfor droplets such that an aerosol is dispensed by the liquid dispense12. However, a pump (e.g., a pressurized container, micropump, screwpump, or piezoelectric pump) could be used to feed the liquid dispenser12 with a fluid (e.g., water if an organic solution is used) to form anemulsion. A pump 54 is coupled to the inlets 18, 20 via a pipe (line orpassage) 56 to provide a core and shell-droplet material. It will beappreciated that only a single pump 54 is illustrated for simplicity,but when producing core-shell droplets, two separate pumps are used; onefor each fluid inlet. Of course, droplets may also be produced using asingle liquid such that only a single pump is required. A compressor 58is coupled to the inlet 30 of the first fluid flow device 14 via a pipe(line or passage) 60 and a compressor 62 is coupled to the inlet 40 ofthe second fluid flow device 16 via a pipe (line or passage) 64.Although not shown in the figure, a suction pump may also be arranged atthe opening 44 of the second fluid flow device 16 to create a negativeair pressure at the opening 44. In this example, the compressors 58, 62feed the apparatus 10 with air, but other gases may be used. Forexample, the compressor 62 may feed the second fluid flow device 16 withair which has a relative humidity of 60% or greater, for example, toreact with the droplets containing cyanoacrylates, for example, exitingfrom the first fluid flow device 14. Furthermore, the compressor 62typically feeds air to the apparatus having a temperate that is higherthan room temperature (i.e., 20° C. to 26° C.). The compressor includesa heating element which is controlled to increase the temperature of theair flowing through the compressor. In this example, the temperature ofreaction fluid is greater than the confinement fluid, which may be usedfor drying droplets, for example. However, it will be appreciated thatthe temperatures of the confinement and reaction flow may be different,such that the temperature of the confinement fluid is greater than thereaction fluid, for example. This may be achieved by increasing thetemperature of the confinement fluid above room temperature whilemaintaining the temperature of the reaction fluid at room temperature,or alternatively, cooling the reaction fluid while maintaining theconfinement fluid at room temperature. In an alternative configurationwhere a negative pressure is created at the outlet 44, the elementlabelled 62 in the figure could be replaced with a heating or coolingdevice only that allows fluid (e.g., air) to be drawn there through andin to the second fluid flow device 16 via inlet 40.

During use, the liquid dispenser 12 is operated and produces acollimated stream of droplets. In this example, the liquid dispenser 12produces a single stream of droplets. The nozzle 25 dispenses dropletswithin the conduit 26 of the first fluid flow device 14, whereby amovement of air (or a flow/stream of air) produced using pump 58 createsa preventative air flow that permits the droplet stream to movegenerally along a trajectory that prevents the droplets from interacting(i.e., adhering) to an interior surface of the conduit 26 of the firstfluid flow device 14. The stream of air also permits the droplets tostabilize after exiting the dispenser. The droplet stream exits theopening 34 of the first fluid flow device 14, and enters the conduit 36of the second fluid flow device 16 and continues to move along a similartrajectory within the flow of air that is provided by the first fluidflow device 14 and the second fluid flow device 16, whereby a movementof air (or a flow/stream of air) produced using pump 62 creates areaction air flow/stream that reacts with the droplets. In this example,the reaction air flow is an air flow at a temperature that is greaterthan room temperature which dries the droplets to form of powder. Thefirst and second air streams will mix via diffusion, for example, suchthat the combined airstream will dry the droplets to formparticles/capsules, or collectively a powder, which are collected at theopening 44. For example, the powder may be collected using differenttypes of reservoirs, cyclones, and/or filters.

Two specific examples are now described using the apparatus 10 describedin association with FIGS. 2, 3 and 4.

In example one, a solution of Shellac (15% w/v) is dispensed from asingle nozzle of the fluid dispenser 12 at a volume flow rate of 30 mL/h(i.e., 8.333*10⁻⁹ m³/s) using pump 54, and a carrier phase is introducedat a pressure of 90 mbar using compressor 50. The velocity of thedroplets exciting the fluid dispenser 12 is 100 m/s and the dropletshave a diameter of 80 μm (+/−5 μm). The air flow from the second fluidflow device 16, produced by compressor 62, is at a volume flow rate of320 L/min (i.e., 0.0053 m³/s) at a temperature of 78.5° C., whichequates to a Reynolds number of 2,182 (i.e., the flow is laminar). Theair flow from the first or preventing fluid flow device 14, produced bycompressor 58, is at a volume flow rate of 60 L/min (i.e., 0.001 m³/s)at room temperature, which equates to a Reynolds number of 1,150 (i.e.,the flow is laminar). In example one, the nozzle 25 of the fluiddispenser 12 is terminated at a distance of 10 mm from the opening orfluid exit 34 of the first fluid flow device 14. The apparatus 10 isoperated according to example one, produced dried particles with a yieldof 90%. It is noted that particles are produced, since only a singleliquid was dispensed from the liquid dispenser 12.

In example two, a solution of Shellac (10% w/v) is dispensed from asingle concentric nozzle of the fluid dispenser 12 at a volume flow rateof 30 mL/h (i.e., approximately 8.333*10⁻⁹ m³/s) using pump 54, and acarrier phase is introduce at a pressure of 90 mbar using compressor 50.In example two, the Shellac is a shell liquid and water is dispensed asa core liquid at a volume flow rate of 1 mL/h (i.e., approximately2.77*10⁻¹⁰ m³/s). The velocity of the droplets exciting the fluiddispenser 12 is 100 m/s and the droplets have a diameter of 80 μm (+/−5μm). The air flow from the second fluid flow device 16, produced bycompressor 62, is at a volume flow rate of 80 L/min (i.e., approximately0.0013 m³/s) at a temperature of 70° C. to 75° C., which equates to aReynolds number of 1,982 (i.e., the flow is laminar). The air flow fromthe first or preventing fluid flow device 14, produced by compressor 58,is at a volume flow rate of 8 L/min (i.e., approximately 0.00013 m³/s)at room temperature, which equates to a Reynolds number of 543 (i.e.,the flow is laminar). In example two, the nozzle 25 of the fluiddispenser 12 is terminated at a distance of 10 mm from the opening 34 ofthe first fluid flow device 14. That is to say that the distancetravelled by the dispensed droplets is 10 mm within the first fluid flowdevice 14. During operation, the apparatus 10, operated according toexample two, produced dried, monodispersed particles. It will beunderstood that as the fluid in the core is just water in this example,the water is evaporated and dried. Therefore, nothing is retained in thecore and the final dried droplet is like a particle without a core. Thisis in contrast to a capsule which includes a core material.

The applicant has determined that the velocity of the confinement fluidin apparatus 10 such that at least 99.5% of the droplets produced usingthe liquid dispenser 12 are recoverable and are able to pass to thesecond, reaction fluid flow device 16, (0.5% of the droplets eitheradhere to the internal walls of the first fluid flow device 14 or becomeagglomerated, for example) is expressed according to the followingformula:

$U_{\min} \geq {k\left( \frac{\mu_{g}^{2}H^{3}}{\rho_{g}^{2}D_{1}^{5}} \right)}^{1/2}$

-   -   where U_(min) is the velocity of the confinement fluid, k is a        constant in the range of 0.2 to 2.0, and includes 0.2, 0.3, 0.4,        0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7,        1.8, 1.9, 2.0 at least, μ_(g) is the viscosity of the fluid fed        by compressor 58 (i.e., the viscosity of the confinement fluid),        H is the distance between a liquid outlet or nozzle 25 of the        liquid dispenser 12 and a fluid exit 34 of the first fluid flow        device 14, ρ_(g) is the density of the confinement fluid, and D₁        is the external diameter of the liquid dispenser 12.

Moreover, to achieve laminar flow in the first, confinement, fluid flowdevice 14, Reynolds number should be less than 4·10³, where Reynoldsnumber is provided by the following known expression:

${4 \cdot 10^{3}} \geq \frac{\rho_{g}{UD}_{3}}{\mu_{g}}$

-   -   where U is the velocity of the confinement fluid and D₃ is the        internal diameter of the first fluid flow device. Accordingly,        the velocity of the confinement fluid can be determined within        the range of:

${4 \cdot 10^{3}} \geq \frac{\mu_{g}}{\rho_{g}D_{3}} \geq U \geq {k\left( \frac{\mu_{g}^{2}H^{3}}{\rho_{g}^{2}D_{1}^{5}} \right)}^{1/2}$

In practical terms, the cross section of the protection device (i.e.,the first fluid flow device) may be chosen/designed to obtain therequired velocity using an air flow rate as low as possible whileconsidering the confinement air is cold and will cool the hot dryingair, which may decrease its drying capacity and avoiding dropletinteraction with the internal walls of the protection device in order toachieve less than 0.5% losses.

FIG. 5 illustrates two images of droplets. In the upper image,polydispersed, agglomerated droplets are illustrated that may beproduced using the known system illustrated in FIG. 1, for example. Thedroplets may have become agglomerated shortly after leaving the fluiddispenser 2. In the lower image, dried, monodispersed particles areillustrated, which are formed from core-shell droplets dispensedaccording to example two described above.

FIG. 6 illustrates a partial cross-section through the apparatusillustrated in FIGS. 2, 3 and 4. In particular, a cross-section throughthe elongate conduit 36 of the second fluid flow device 16, the elongateconduit 26 of the first fluid flow device 14 and the fluid dispenser 12.The figure illustrates schematically the fluid flow from each of thedevices and how it interacts. As can be seen from the figure, the fluidexiting the fluid dispenser 12 is travelling at a different velocity tothe fluid flow from the first fluid flow device 14, such that there maybe sheering where the two fluids meet, which may cause local turbulence.However, the first fluid flow device 14 is dimensioned so as to preventany disturbance to the droplet stream exiting the fluid dispenser 12.Similarly, the fluid exiting the first fluid device 14 is typicallytravelling at a difference velocity to the fluid flow exiting from thesecond fluid flow device 16, such that there may be sheering forceswhere the two fluids meet, which may cause local turbulence. Anydisturbance between the first and second (i.e., the confinement andreaction) fluid flows is less significant than any disturbance betweenthe droplets exiting the fluid dispense 2 and the first fluid flow,since once the droplets have exited from the first fluid flow and intothe second fluid flow, they are already typically stabilized. Once thefirst (confinement) fluid flow has exited the first flow device 14 intothe second (reaction) fluid flow, the two fluids will gradually mix bydiffusion as the fluids flow from the left to the right in theorientation shown in the figure, such that the increased temperature ofthe second fluid flow will effectively increase the temperature of thefirst fluid flow and dry the droplets to form a powder.

FIG. 7 illustrates a cross-section through a flow focusing nozzleaccording to a second embodiment of the invention. The nozzleillustrated in FIG. 7 forms the distal end of the liquid dispenser 12illustrated in FIGS. 2 and 3. In this regard the duct 24 forms the outerchannel or conduit of the nozzle. The nozzle is concentrically arrangedand includes a first inner channel or fluid passage 51 (i.e., an outerfluid dispenser) arranged concentrically within the duct 24 and a secondinner channel or fluid passage 53 (i.e., an inner fluid dispenser)arranged concentrically within the first inner channel 51. Although notillustrated in FIG. 7, the channel or duct formed between the duct 24and the first inner channel 51 is coupled to the fluid inlet 22illustrated in FIGS. 2, 3 and 4 for feeding a carrier fluid 59, thechannel or duct formed between the first inner channel 51 and the secondinner channel 53 is coupled to the second fluid inlet 20 illustrated inFIGS. 2, 3 and 4 for feeding a shell-droplet fluid 55, and the secondinner channel 53 is coupled to the first fluid inlet 18 illustrated inFIGS. 2, 3 and 4 for feeding a core-droplet fluid 57. Each of the firstand second inner ducts 51, 53 are terminated within the duct 24 suchthat fluid exiting from these inner ducts interacts with the carrierfluid 59 within duct 24. The nozzle, orifice or outlet 25 allows thepassage of the combined core-shell fluid 57, 55 and the carrier fluid 59out of the device first as a stream, which subsequently forms droplets61. It will be appreciated that the first inner wall 51 is optional, andis not typically used if a single fluid is used to generate the droplets(i.e., the generated droplets are not core-shell type droplets). Each ofthe fluids 55, 57, 59 illustrated in FIG. 7 may be a gas or a liquid.

During operation, fluid is fed to each of the fluid inlets and travelsalong each respective duct to the orifice 25. As the fluids 55, 57 exitthe first and second inner ducts 51, 53 they do not typically mix, suchthat the first fluid 55 forms an outer shell to the second fluid 57. Thecarrier fluid 59 accelerates the first and second fluids 55, 57 togenerate a monodispersed beam of droplets.

The applicant has determined that a collimated stream of droplets havinga geometric standard deviation of the disbursement of droplets from thecollimated stream of droplets (i.e., a measure of the spread of dropletswith respect to the droplet stream) of less than 1.6 can be achievedwith a nozzle configured to according to the expression:

$\left( \frac{D^{2}\Delta\; P^{2}\rho_{l}^{2}Q_{l}}{\sigma\;\mu_{l}^{3}} \right)^{1/4} \leq \frac{1 + {0.5\left( \frac{\rho_{l}}{\rho_{d}} \right)}}{1 + {0.018\left( \frac{\mu_{l}}{\mu_{d}} \right)}}$

-   -   where D is the nominal diameter of the discharge orifice (i.e.,        orifice 25), AP is the pressure of the carrier fluid, ρ_(l) is        the density of the fluid being dispensed, Q_(l) is the flow rate        of the fluid being dispensed from the first fluid dispensing        device, σ is the surface tension between the fluid being        dispensed and the carrier fluid, ρ_(d) is the density of the        carrier fluid, and μ_(d) is the viscosity of the carrier fluid.        In the nozzle illustrated in FIG. 7, two fluids are used to form        a core-shell type droplet. In this case, the density of the        fluid being dispensed (ρ_(l)) is taken to be the greatest        density of the two fluids being dispensed. In the nozzle        illustrated in FIG. 7, two fluids 55, 57 are dispensed to form        core-shell droplets, such that the density and viscosity values        (i.e., ρ_(d) and μ_(d)) are typically of the dispensed fluid        with the greatest fluid velocity, which is typically the shell        forming fluid 55.

Preferably, a geometric standard deviation of the disbursement ofdroplets from the collimated stream of droplets of less than 1.4, andmore preferably less than 1.3 can be achieved using a nozzle configuredaccording to the above-expression. The angle of the fluid dispensed froma nozzle (angle 63 illustrated in FIG. 7) is preferably less than orequal to 5 degrees.

FIG. 8 illustrates an apparatus 70 for spray drying according to asecond embodiment of the invention. Apparatus 70 is similar to apparatus10 in that it includes the same second flow device 16. Apparatus 70 alsoincludes a similar first fluid flow device 14′ to that described inassociation with apparatus 10, except the opening 74 of the first fluidflow device 14′ is greater than the opening 32 of the first fluid flowdevice 14 to accommodate a larger fluid dispensing device or fluiddispenser 72. The fluid dispensing device 72 is larger in diameter thanthe fluid dispenser 12, since it includes multiple nozzles to producemultiple droplet streams, as is described in association with FIG. 10.The apparatus 70 is operated in the same manner as apparatus 10, exceptfor the fluid dispenser 72, which is described in association with FIG.10.

The apparatus 70 illustrated in FIG. 8 includes a multi-nozzledispensing device 72. Using the expression for a single nozzledispensing device presented above, it is possible to predict thevelocity of the confinement fluid required in a multi-nozzle dispensingdevice to recover at least 99.5% of the droplets produced using themulti-nozzle dispensing device. This is done by replacing the value of Hin the previously presented expression with the expression H/(1-D₂/D₃),where H is the distance between a liquid outlet or nozzle of the liquiddispenser 72 and a fluid exit of the first fluid flow device 14′, D₂ isthe smallest diameter of a virtual circle containing the centres of thedischarge orifices/exits of the nozzles in the multi-nozzle dispensingdevice 72 and D₃ is the internal diameter of the first fluid flow device14′. The following expression is obtained:

$U_{\min} \geq {k\left( \frac{\mu_{g}^{2}H^{3}}{\rho_{g}^{2}{D_{1}^{5}\left( {1 - {D_{2}/D_{3}}} \right)}^{3}} \right)}^{1/2}$

-   -   where U_(min) is the velocity of the confinement fluid, k is a        constant in the range of 0.2 to 2.0 and includes 0.2, 0.3, 0.4,        0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7,        1.8, 1.9, 2.0 at least, μ_(g) is the viscosity of the        confinement fluid, H is the distance between a liquid outlet or        nozzle of the multi-nozzle dispensing device 72 and a fluid exit        of the first fluid flow device 14′, ρ_(g) is the density of the        confinement fluid, D₁ is the external diameter of the        multi-nozzle liquid dispenser 72, D₂ is the smallest diameter of        a virtual circle containing the centres of the discharge        orifices/exits of the nozzles in the multi-nozzle dispensing        device 72 and D₃ is the internal diameter of the first fluid        flow device 14′.

A specific example is now described using the apparatus 70 described inassociation with FIG. 8.

In example three, a solution of Arabic Gum (10% w/v) is dispensed from amultiple nozzle including 8 nozzles of the fluid dispenser 72 at avolume flow rate of 30 mL/h per nozzle (i.e., approximately 8.333*10⁻⁹m³/s), and a carrier phase is introduce at a pressure of 85 mbar. Thedroplets have a diameter of 82 μm (+/−5 μm). The air flow from thesecond fluid flow device 16 is at a volume flow rate of 550 L/min (i.e.,approximately 0.0092 m³/s) at a temperature of 70° C. to 75° C., whichequates to a Reynolds number of 4,330 (i.e., the flow is NOT laminar).The air flow from the first or preventing fluid flow device 14′ is at avolume flow rate of 50 L/min (i.e., approximately 0.00083 m³/s) at roomtemperature, which equates to a Reynolds number of 982 (i.e., the flowis laminar). In example three, the nozzles 25 of the fluid dispenser 72are terminated at a distance of 10 mm from the opening of the firstfluid flow device 14′. That is to say that the distance travelled by thedispensed droplets is 10 mm within the first fluid flow device 14′. Thefirst fluid device 14′ has a diameter of 60 mm. During operation, theapparatus 70, operated according to example three, produced dried,monodispersed particles.

FIG. 9 illustrates an apparatus 80 for spray drying according to a thirdembodiment of the invention. Apparatus 80 is similar to apparatus 70 andincludes four fluid flow devices 14′, each having a fluid dispenser 72.The second fluid flow device 16′ is also similar to the second fluidflow device 16 of apparatus 10, but as can be seen it is greater indiameter and includes four openings 82 to accommodate the four fluidflow devices 14′. The fluid dispensing device 72 is described inassociation with FIG. 10. The apparatus 80 is operated in the samemanner as apparatus 70.

FIG. 10 illustrates a multi-nozzle fluid dispenser 72 which may be usedwith the second and third embodiments of the invention. The multi-nozzlefluid dispenser 72 includes 8 nozzles 25 in this example. Each of thenozzles 25 includes an associated duct 24, first inner channel 51, andsecond inner channel 53, as is illustrated in FIG. 7. These are notillustrated in FIG. 10 for simplicity. Each of the ducts for each nozzleis also coupled to an associated compressor or pump. It will beappreciated that the same compressor/pump may feed all of the respectiveducts 24, first inner channels 51, and second inner channels 53depending on the type of droplets being formed.

It will be appreciated that the dimensions provided for the apparatusdescribed herein are only examples and should not be considered to limitthe scope of the invention. In this regard, the outer diameter of thefluid dispenser may range from 5 to 100 mm. Furthermore, the internaldiameter of the first fluid flow device may range from 5.5 mm to 1000 mmand the internal diameter of the second fluid flow device may range from50 mm to 2000. Moreover, the ratio between the outer diameter of thefluid dispenser and the internal diameter of the first fluid flow deviceis from 1:1.1 to 1:100. It is noted that the values for density andviscosity mentioned herein are with respect to room temperature (i.e.,20° C. to 26° C.). The viscosity mentioned herein is a dynamicviscosity, and the density and viscosity values mentioned herein havebeen determined using the standard test method for dynamic viscosity anddensity of liquids by Stabinger Viscometer, for example, in accordancewith the ASTM standards.

The examples are typically illustrated in a vertical orientation suchthat the droplets are introduced at the top and exit at the bottom ofthe apparatus. This is the typical orientation of the apparatus duringuse.

In the examples described herein, the confinement flow device is asingle elongate conduit. However, this can be increased to includemultiple elongate conduits arranged concentrically. The multipleelongate conduits will also have different lengths, such that theshortest conduit will terminate within the next longest conduit and soforth. Thus, a nested, or telescopic, confinement device is achieved.

The invention claimed is:
 1. An apparatus for generating droplets,comprising: a liquid dispenser comprising a liquid outlet adapted togenerate a collimated droplet stream; a first fluid flow deviceconfigured to generate a confinement fluid flow for confining atrajectory of the droplet stream therein, the first fluid flow devicecomprises an outlet arranged to allow the droplet stream to exittherefrom, and further comprising a first conduit wherein the liquidoutlet is arranged so as to dispense the droplets within the firstconduit of the first fluid flow device; and a second fluid flow deviceconfigured to generate a reaction fluid flow for reacting with dropletsin the droplet stream, the second fluid flow device comprising a secondconduit, wherein the first fluid flow device is arranged within thesecond fluid flow device so that the droplet stream exists in the secondconduit of the second fluid flow device.
 2. The apparatus of claim 1,wherein the liquid dispenser is concentrically arranged within the firstfluid flow device, and/or the first fluid flow device is concentricallyarranged within the second fluid flow device.
 3. The apparatus of claim1, wherein each of the confinement fluid and/or the reaction fluid isair.
 4. The apparatus of claim 1, wherein the first and/or second fluidflow devices comprise an elongate conduit, and the elongate conduit arecircular.
 5. The apparatus of claim 1, wherein the liquid outletcomprises a plurality of nozzles, each configured to generate a dropletstream.
 6. The apparatus of claim 1, wherein the temperature of thesecond fluid flow is greater than the temperature of the droplet streamand/or the confinement fluid flow.
 7. The apparatus of claim 1, whereinthe reaction comprises hardening the droplets in the droplet stream. 8.The apparatus of claim 1, wherein the liquid dispenser is configured togenerate the droplet stream using flow focusing.
 9. The apparatus ofclaim 1, wherein the first fluid flow device is dimensioned to produce alaminar fluid flow.
 10. A system comprising the apparatus of claim 1 anda first fluid flow generating device coupled to the first fluid flowdevice and configured to provide the confinement fluid to the firstfluid flow device.
 11. The system of claim 10, wherein the first fluidflow generating device and the first fluid flow device are configuredaccording to the following expression:$U_{\min} \geq {0.2 \cdot \left( \frac{\mu_{g}^{2}H^{3}}{\rho_{g}^{2}D_{1}^{5}} \right)^{1/2}}$where: U_(min) is the velocity of the flow rate of the confinementfluid; μ_(g) is the viscosity of the confinement fluid; His the distancebetween the liquid outlet of the liquid dispenser and the outlet of thefirst fluid flow device; ρ_(g) is the density of the confinement fluid;and D₁ is the external diameter of the liquid dispenser.
 12. A methodcomprising: generating a droplet stream from a liquid dispensercomprising a liquid outlet adapted to generate a collimated dropletstream; generating a confinement fluid flow within a first fluid flowdevice to confine the trajectory of the droplet stream, wherein thedroplet stream is generated within the confinement fluid flow and exitsfrom an outlet of the first fluid flow device, wherein the liquid outletis arranged so as to dispense the droplets within a first conduit of thefirst fluid flow device; and generating a reaction fluid flow within asecond fluid flow device for reacting with droplets in the dropletstream, wherein the confinement fluid flow is generated within thereaction fluid flow, and wherein the droplet stream exits a secondconduit of the second fluid flow device.
 13. The method of claim 12,wherein the reaction fluid flow hardens the droplets to form particlesand the method comprises collecting the particles at an outlet of thesecond fluid flow device.
 14. The method of claim 12, wherein a minimumvelocity of the confinement fluid flow is represented by the expression:$U_{\min} \geq {0.2 \cdot \left( \frac{\mu_{g}^{2}H^{3}}{\rho_{g}^{2}D_{1}^{5}} \right)^{1/2}}$where: U_(min) is the velocity of the flow rate of the confinementfluid; μ_(g) is the viscosity of the confinement fluid; H is thedistance between the liquid outlet of the liquid dispenser and theoutlet of the first fluid flow device; ρ_(g) is the density of theconfinement fluid; and D₁ is the external diameter of the liquiddispenser.
 15. A method of generating a collimated stream of dropletsfrom a flow focusing nozzle comprising a discharge orifice, a firstfluid dispenser comprising a liquid outlet adapted to generate thecollimated stream of droplets, wherein the liquid outlet is arranged soas to dispense the collimated stream of droplets within a first conduitof the first fluid dispenser, and a second fluid dispenser, wherein thestream of droplets exits a second conduit of the second fluid dispenser,and wherein the second fluid dispenser is arranged to accelerate, with acarrier fluid, a fluid being dispensed from the first fluid dispenserout of the discharge nozzle, the method comprising configuring the flowfocusing nozzle to obtain a geometric standard deviation of thedisbursement of droplets from the collimated stream of droplets of lessthan 1.6 according to the expression:$\left( \frac{D^{2}\Delta\; P^{2}\rho_{l}^{2}Q_{l}}{\sigma\;\mu_{l}^{3}} \right)^{1/4} \leq \frac{1 + {0.5\left( \frac{\rho_{l}}{\rho_{d}} \right)}}{1 + {0.018\left( \frac{\mu_{l}}{\mu_{d}} \right)}}$where D is the nominal diameter of the discharge orifice, AP is thepressure of the carrier, ρ_(l) is the density of the fluid beingdispensed, Q_(l) is the flow rate of the fluid being dispensed from thefirst fluid dispenser, σ is the surface tension between the fluid beingdispensed and the carrier fluid, μ_(l) is the viscosity of the fluidbeing dispensed, ρ_(d) is the density of the carrier fluid, and μ_(d) isthe viscosity of the carrier fluid.
 16. The method of claim 15, whereinthe flow focusing nozzle is configured to obtain a geometric standarddeviation of the disbursement of droplets from the collimated stream ofdroplets of less than 1.4.
 17. The method of claim 15, wherein the flowfocusing nozzle is configured to obtain a geometric standard deviationof the disbursement of droplets from the collimated stream of dropletsof less than 1.3.
 18. The method of claim 15, wherein the first fluiddispenser comprises an inner fluid dispenser configured to dispense acore fluid and an outer fluid dispenser configured to dispense a shellfluid and wherein ρ_(l), Q_(l), and μ_(l) are of the shell fluid.